1. Field of the Invention
The present invention relates to a heart stimulating device having the capability to detect the occurrence of fusion and pseudofusion heart beats, i.e. delivery of electrical stimulation pulses simultaneously or with a small separation in time from spontaneous heart beats.
2. Description of the Prior Art
Fusion beats generally refers to a situation where atrial or ventricular depolarization starts from two different positions in the heart. Such a situation arises when a natural heart beat occurs simultaneously with an electrical stimulation pulse from a heart stimulating device and they both contribute to the depolarisation of a heart chamber.
Pseudofusion beats generally refers to a situation where atrial or ventricular depolarization starts spontaneously in the heart and a stimulation pulse is delivered subsequently and does not contribute to the depolarization.
In demand pacemakers or other heart stimulating devices that aim to electrically stimulate a patient's heart only in the absence of normal intrinsic activity, fusion beats or pseudofusion beats present a particular problem, since intrinsic events are to be favoured and stimulation energy is to be saved until really needed. In that way the longevity of a battery powered heart stimulation device is improved.
Some terminology used conventional is explained below. Impedance: The total resistance to the flow of electrical current through a conductor, including resistance produced by tissue and electronic components including resistors, capacitors and coils. In pacing, impedance and resistance are sometimes used interchangeably, but strictly speaking, impedance involves a time-dependent relationship between voltage and current, whereas resistance is a fixed number. Resistance and impedance is usually stated in ohms.
IEGM: An abbreviation for intracardiac electrogram. IEGM signals are emitted by active cardiac tissue and sensed through electrodes placed on or within the heart. PA0 QRS or QRS complex: The ventricular depolarization as seen on the electro cardiogram or in the IEGM signals. PA0 Intrinsic: Inherent or belonging to the heart itself. An intrinsic beat is a naturally occurring heart beat. PA0 R wave: an intrinsic ventricular event. R refers to the entire intrinsic QRS complex. PA0 Evoked response: The electrical activation of the myocardium by a pacemaker output pulse. The ability of cardiac tissue to respond depends on its activity state. PA0 Ventricular fusion beat: the pacemaker impulse appears close to a spontaneous QRS complex and partly contributes to the ventricular depolarization. PA0 Ventricular pseudofusion beat: the pacemaker impulse appears within a spontaneous QRS complex and does not contribute to the ventricular depolarization. PA0 Atrial fusion beat: the pacemaker impulse appears close to a spontaneous P wave and partly contributes to the atrial depolarization. PA0 Atrial pseudofusion beat: the pacemaker impulse appears within a spontaneous P wave and does not contribute to the atrial depolarization.
A fusion or pseudofusion beat, either in a ventricle or an atrium, may be referred to as simply a fusion beat in contexts where problems related thereto are similar to those of regular fusion beats.
In conventional pacemakers, pulses may be delivered although intrinsic heart activity is present at a rate close to that of the pacemaker. Moreover, backup pulses may by delivered by the pacemaker for safety reasons when an evoked response is rendered undetectable by a fusion or pseudofusion beat. In these and other cases, the detection of evoked response is disturbed by intrinsic heart activity. Further, in order to detect lead breakage or other anomalies, such a pacemaker may be provided with means to determining a measure of the impedance of the lead arrangement by measuring voltage at leading and trailing edges of a delivered stimulation pulse.
Conventional pacemakers still need improvement to effectively overcome the problem of consuming the battery energy prematurely due to failure to favor present intrinsic activity in the heart. Detection of fusion and pseudofusion beats present a general problem in the art.